Gas permeable membrane for air-depolarized cell



Aug. 14, 1956 P. A. MARSAL 2,759,038

Y GAS PERMEABLE MEMBRANEFOR AIR-DEPOLARIZED CELL Filed Feb. 10, 1954 INVENTOR PAUL A. MARSAL United States Patent GAS PERMEABLE 'NIEMBRANE FOR AIR-DEPOLARIZED CELL Paul A. Marsal, Rocky River, Ohio, assignor to Union Carbide and Carbon Corporation, a corporation of New York Application February 10, 1954, Serial No. 409,435

9 Claims. (Cl. 13G- 136) This invention relates to improvements in primary cells of the air-depolarized cathode type, and more particularly concerns gas permeable membrane covers for airdepolarized cells.

In the conventional primary battery of the air-depolarized cell type, as for example one comprising an anode of zinc, a porous, active, carbon cathode and an electrolytic solution therebetween, the air takes part in the battery reaction in the following manner. yIn cell containers `of the type illustrated in the patent to P. A. Marsal et al., 2,597,116, issued May 20, 1952, suitably located ports or openings may be provided in the cell walls -to allow air to contact exposed portions of the carbon cathode. ln other cell constructions, for example conventional air-depolarized cells such as those used for railway signal service, the entire upper surface of the carbon cathode may be directly exposed to the atmosphere, `or the atmospheric air may traverse an inner chamber in the cell to reach the cathode breathing surfaces. In all instances, lthe carbon sorbs the oxygen, and through a process of dilfusion land migration, makes it available in active form at the electrode-electrolyte interface. The depolarization or cathodic reduction reaction then converts the oxygen into a peroxy compound, which under ordinary conditions of cell use is quickly dissipated.

The functioning of the active carbon cathodes may be impaired in a number of ways. lf the humidity of the atmosphere is relatively high, some of Kthe contained moisture in the air may be retained in -the carbon pores, thereby reducing :the further sorption of atmospheric air. The moisture and water vapor reaching the interior of the cell dilutes the electrolyte, increases the electrolyte volume, and thus creates a leakage hazard. Conversely, if the humidity of the ambient air is low, moisture will escape from the cell. Where Athe cell is of the so-called dry type, any considerable moisture loss -will cause electrolyte shrinkage, and thus limit lthe useful life of the cell. Also, there is a tendency for some of the carbon dioxide in the air to enter the cell where it is absorbed by the caustic electrolyte with loss `of electrolyte eifectiveness due to lower cconductivity, lower oxide solubility and greater tendency to form adherent oxide films.

For these and other reasons, it is important that the moisture gain or loss of an air-depolarized cell be reduced without obstructing the passage of atmospheric oxy- .genl into the cell to fulll the depolarization requirements i ice cient oxygen from the air so as to eiciently and effectively depolarize the cell.

Other objects, features and advantages of the present invention will be lapparent from the following detailed description of a preferred embodiment thereof, taken in conjunction Iwith tthe accompanying drawing, in which the ligure is a front elevational View partly in section, illustrating the principles of the present invention.

In its broadest aspects, the principles and features of the .present invention are involved in the reduction of moisture transmission through the porous carbon cathode lof an air-depolarized cell, and the maintenance of a llow of oxygen from the air into the interior of the cell for the purpose of depolan'zing the carbon electrode. Brley, referring to Fig. 1, a conventional dry type airdepolarized cell 10 may comprise an impervious shell casing 11 having therein an anode 12 such as amalgamated zinc, a carbon cathode 13 and some form of electrolyte solution 14, such as sodium hydroxide.

Vent openings or breather ports 1S are provided in the casing to allow the cell 10 to breathe. Specically, these openings permit access of atmospheric air into the cell. The oxygen of the air is occluded by the carbon cathode 13, and is thereafter utilized during cell discharge to perform its depolarization function at the carbon cathodeelectrolyte interface.

ln the normal course of operation, a number of factors will tend to impair the functioning of the cell. One of the most serious yof :these concerns a change in the moisture content of the cell. This may be manifested in several Ways. First, the water from the electrolyte solution 14 may evaporate due to low humidity conditions in the atmosphere, causing dessication in the cell. Second, ythe atmospheric humidity may be so high as to cause condensation of water vapor in the pores of the activated carbon cathode 13. Or third, a high concentration of water vapor in the carbon may lead to vapor diffusion into the electrolyte causing dilution of the electrolyte, and, as a result of the consequent increase in the electrolyte volume, create a leakage hazard in the cell. In each instance, the change in moisture content of the cell will seriously aifect its useful life.

One of the diiliculties involved in reducing the movement of moisture in a cell of the airdepolarized type lies in the fact that a reduction in the accessibility of moisture to the cell is usually accompanied by a corresponding reduction in the availability of depolarizing air. Naturally a reduction in the flow of air will correspondingly limit the rate of discharge of the battery.

It has been discovered that certain materials are impermeable to moisture and permeable to gases. By covering the vent openings in an air-depolarized cell with a covering 16 made from any of these gas permeable materials, the -tendency for the moisture content of the cell to change in accordance with atmospheric conditions is substantially retarded, and the accessibility of air to the interior of the cell is not seriously impeded. A number of materials may be suitable for the purpose at hand to produce fthe vdesired effects. Plastic compositions such as polyethylene, ethyl cellulose, or the like, have been found to produce satisfactory results.

Moreover, according to the present invention, a lm of polyethylene exhibits the further characteristic of selective s-orption of the oxygen component of lthe air to the substantial exclusion of nitrogen and carbon dioxide present in the air. Tests have shown Ithat .a poly ethylene hlm passes oxygen rather readily, but is only one-third as permeable to nitrogen.

Tests have shown that the thickness of the material to be used as a moisture screen and air transmitter should not be so great as to render it impermeable to both air and moisture, nor so thin as to allow the sorption of all othe. gasf-constituentsdn-thef atmosphere. On the other hand,..maximum..thickness must .be .such..as.not.to-oer any serious resistance to air.ow, so that an amount of air for depolarization purposes proportional to the currenbfdemandszofithefcell enters throu'ghithecellvents. iOn the basis of experimental results, atpolyethylenealmghavingr ati-thickness of i. 2003 .inch hasfbeen'wfound toagive .satisfactoryresults.

i Inzzthe ,present invention; asia: :result .oftheselective permeable :actionsof` ,fthe .polyethylene membrane. covering .the breatherxportsaoan air-depolarized primary: cell, an oxygen enrichedstreamiof'A air-isA available .for .depolarization .'purposes. -This 4.enriched-.f air.; affords a,- ymore eilicientruseof; the: air as azdepolarizeruthan ris :possible in lthe-:conventional iair-depolarized; cell operating y without a. plasticf.vent\covering. .Asia-consequence, higher and morerzsustained voltageelevels are raorded with. little if any moisture -transferiintolor .outgof the cell.

-Typical .te'st resultszobtained .fromV air-.depolarized cells havingiplasticrmembranesfaccording to this invention, in comparison with control.cellshaving.conventionalbreather spacesecommunicatingdrectlyi-.with the.: atmosphere I are set forth inthey followingftable. 11n thesetests, ltheicells .were Istored for theaperiods indicated= overca'lcium chloride inea. dessicator-.;having..a nitrogen: atmosphere,A andtheir moisture loss wasidetermined attheiend of -such periods.

Percent moisture lost kat: 70 F.

. Control (no Polyethylene Y film membrane) 3=weeks -mission'fis .much-highenb'eingata times nas much as-six :times more2 effective '.than polyethylene.

ein; practica,- the .fgasepermeablen lrn may :be suitably ttedr'overfr-.the surface.- of the:4 .cellfin .which thev vent .is

.provided iiPreferably.polyethylene tubing or other suitable plasticfvmaterialfis .shrunk onto fthe. cell ysurface so as ;t0.^.cov.er the;.breathing;holes of thev cell. ..-Frorrr.1the;.above descriptionitzwill, therefore,-be underfstodthatiaceordingy tto:` this; invention, .plastic .lm materialsare providedzoverthe breather-ports -of air-depolar- :zecl'y .cells fornimpedingvvapor .transmission in both f direc- .tions .throughasuch :cel1s,;;.and: improving :depolarization .action;;therewithin,; all .to; 'the zend-f that A.longer cell' life is feiected.

vIt isetofbe'Y understood that, variations andmodications may be effected withoutedepartingffrom vthegnovelcon- .ceptstof therpresent invention.

l. .-In fan i;airsdepolarizedazcell: :having :breather spaces .f-for; the,l passage :of airr into'; Tthe interiorz'of the cell, Vthe -improvement f; comprising gan oxygen-permeable, `-xwateriimpermeableplastic.zfmembrane :covering A.said 'breather space.

2. In an air-depolarized cell including an impervious ....outer..cell.wall,...an,active.,porous.,.carbon,cathode.and.an

anode Within said cell, anrelectrolyte solution between said anode and said cathode, and vent openings provided in said cell wall for the passage of air to said porous cathode, the improvement comprising a moisturerepellent, oxygen-permeable:plastic film mounted on the cell over.. said .vent openings.

3. In an airdepolarized cellrhaving' breather spaces for the passage of air into `the interior of -the cell, a thin polyethyienemembranecovering v*said ,breather .space;;sa'id membrane vbeing. selectively @permeable yto..'oirygenfimsaid air and substantially impermeable to vapor-transrnission in bothfdirections in isaid cell.

4. In an air-depolarized cell including an impervious outer cell Wall,=,an.active porousucarbonfcathode and an anode Within said cell, an electrolyte solution between said anode and said cathode, and vent openings provided zine-said; ...ee'llff wall;. tor; .thenpassage@Soit-.ain` stof; said )porous cathode,ftherimprovementgcompnising.aspolyethylendilm -mountcdf-Onethe:ieellnover esaidztv-entV openingsnrsaidslm having selective oxygen transmissibility qualitiesandzbe- .ingsubstantiallyfmoistuw-repellent.

5 ,11n-rang. airfdepolarized cellV provided with :openings gfor..the admissionz-oi,.depolarizingaair, polyethyleneilm means forpreventing moisture isorptionzinto; orfout Aoisaid cell iperlings;1 saidz-'meansg-being; characterized -by1sselective -ftransmissibility qualities.- e of." preventing a the. sorption I. .-of carbondioxide.'and nitrogen ffrom'the. air, :and simultanetously` occluding the roxyzgenfromlhe air.

6. ln-1; an airfdepolarized'- cell ',having :breather ports; Y for the'passage;o.airfintothe interior of the cell, athinrethyl lcellulose membrane covering said breather,:ports,;.said membrane :being -.-selectively permeable to oxygen iin; said .;air, .andgsubstantiallyimpermeable;to;water vapor in;said .air andsaid cell.

- 7. lm-am,airfdepolarized cell; :including qanrimpryiou outer cell wall, an. `active porous ca-rbonf cathode-I and'y an zanode within-1 said veell, .an electrolyte =solution sbetween .said'anodez and: said cathode,-.andfvent.openingsaprovided .in .said.- eellwvall. for ithe5passagefoffairftoe said/porous .cathodefthe.improvement comprisingani ethyl cellulose lruY ,i-mountednon the. cell fover saidventr.-openingsftsaid .lm having selective .oxygen transmissibility .qualitiesand Lbeing substantially moisturey repellent.

8.1 dmanr; air.depolarizeda celly provided ...Withppenings forfthegadmission of :depolarizing air, ethylicellulose film means.. forfpreventing: moisture Wsorption 1into lorX out .of @said .icelL.y said .means beings characterized -.by g-sfeleetive transmissibility Y.lqualities of; preventing t'he fsorptionaof carbon.dioXideandnitrQgeni from4 the-.air,:and. simultanezously .-occluding .the zoxygenfrom the air.

9.- Inv an.l air-depolarized. Vcellgprovidedwvvith @breather .ports forl the. admission..- of s,depolarizingeaira Aplastictiilrn fmeans covering-saidgfports.for preventing..the ,movement .oiwaten vapor.in.both.directions `through l.said ports, said meansehayingiathickness such as .to render.4 said .film means ;.se1ectively lpermeable gtoeoxygemin itheftainfiand substantially.*Jessppermeable to-znitrogenrandggcarbongdi- .oxidefin ithe'f air.

"1g-969,630 Sprague et" al Augf7f 1934 2,571,927 Neumann et al. Oct. 16,1951 A1257-2,9'l8 Fisher etal. "Oct, 30, i951 2,1597-, l'el6 Ma-rsal etal MayZO; -1952,

2,597,117 Schumacher etal. -'May 20,'"1952 256323032 Winckler "-Mar."` 17, 1953 l'236415623 U`Winckler et al -Inne"'9, 1953 

7. IN AN AIR-DEPOLARIZED CELL INCLUDING AN IMPREVIOUS OUTER CELL WALL, AN ACTIVE POROUS CARBON CATHODE AND AN ANODE WITHIN SAID CELL, AN ELECTROLYTE SOLUTION BETWEEN SAID ANODE AND SAID CATHODE, AND VENT OPENINGS PROVIDED IN SAID CELL WALL FOR THE PASSAGE OF AIR TO SAID POROUS CATHODE, THE IMPROVEMENT COMPRISING AN ETHYL CELLULOSE FILM MOUNTED ON THE CELL OVER SAID VENT OPENINGS, SAID FILM HAVING SELECTIVE OXYGEN TRANSMISSIBILITY QUANTITIES AND BEING SUBSTANTIALLY MOISTURE REPELLENT. 